Filter elements for filtration of gaseous and liquid fluids are readily available on the market in a variety of embodiments. These filter elements are inserted into a filter housing and are used to clean supplied fluid that is laden with dirt particles, for example, in the form of a hydraulic medium, by using a mat-type or sheet-type filter web. The cleaned fluid from the filter housing is returned to the fluid circuit, in particular to a hydraulic circuit. In principle, the operational reliability of such hydraulic circuits and of the components included therein depends largely on the proper composition of the pertinent fluids circulated therein. Especially in higher-grade systems for economic reasons, suitable filter devices need to be provided for the media and fluids under consideration to be able to reliably remove impurities that occur in operation. In the case of the filter elements mentioned, assembling mat-type or sheet-type filter webs from different filter materials and folding or pleating at least part of the surface areas thereof is generally known to produce folded or pleated filter mats therefrom, which are placed in particular around a support tube provided with passages. To protect them from damage, such filter elements are provided on their outer periphery with suitable protective devices, such as a fine wire mesh that follows, in particular, the folding pattern of the filter mat. In lieu of finely woven wire mesh, placing fiber structures, such as fabric bands, around the outer periphery of such a filter element is known to thereby obtain protection against damage to the mat-type or sheet-type folded or pleated filter mats. Joining of the filter mat to the wire mesh or even to fabric bands is elaborate and expensive, and does not guarantee in every case that the filter mat is protected from mechanical damage. Such additionally applied protective devices also tend to become detached or to delaminate, in particular when the direction of fluid inflow at the filter element reverses, for example in the case of a backwash process. The known protective devices, such as the wire mesh mentioned or textile fiber structures, such as bands, furthermore have the shortcoming that the flow and filtration characteristics thereof, as compared to sensitive filter materials made of polyester, fiberglass or paper web, are very different and sometimes inferior. Such protective devices generally do not provide any defined pre-filtering effect.
DE 10 2005 014 360 A1 shows and describes a filter element having a filter material that is folded in a star shape and has individual filter folds. At least one fluid-permeable support extends at least partially in the space between two adjacent filter folds, at the inner or outer periphery relative to the filter folds. The support is provided in particular with filter-active substances or is itself composed of these filter-active substances and can be used as a filtering aid. The fluid-permeable support has, in particular, a basic structure in the fashion of a support tube that surrounds the filter material of the filter element on the inner or outer periphery thereof. Embedded into the basic structure are filtering aids, such as bentonites, perlites, activated carbon, diatomaceous earth, and the like, which, as filter-active substances, are capable of preventing the filter element from becoming clogged with sludge, in that components in the fluid that are harmful to the fluid or to system elements are reliably separated.
WO 01/37969 shows and describes a fixing band, in particular in the form of an adhesive tape, that can be placed around a pleated filter medium to stabilize and fix individual filter folds of the pleated filter medium at their prescribable spacing relative to one another. This structure ensures that a predefined filter surface of the filter medium is durably provided. However, pre-filtration is not possible with this known solution.
These known filter elements are characterized in particular in that any functionality imparted thereto is realized by a manufacturing or work step using individual components that is not optimal in terms of its production time or material consumption.
From DE 693 16 647 T2, a filter element is disclosed that has a porous, thick-walled, integral, self-supporting resin-impregnated and resin-bonded fibrous, tubular filter structure, having:                a hollow core;        an inner shell made of a first large-pore porous medium adjacent to the hollow core; and        an outer shell made of a second small-pore porous medium, finer than the first porous medium and adjacent to the inner shell. The first and second porous media are resin-impregnated and resin-bonded. Fluid to be filtered in operation flows radially inward through the tubular filter structure.        
To improve the quality grade of this known filter element, an incrementally graded porosity is provided, in such a manner that the pores are most numerous on the outside of the filter element, with the dimensions thereof being at their smallest there. Preferably, provisions are made to incorporate on the outside of the filter element, and thus in the outer filter jacket, depressions, as structures that then constitute along the longitudinal axis of the filter element individual rings extending parallel to one another.
For the manufacture of the known filter element, a fibrous material is mixed with water or with another suitable dispersant, to form a slurry. Then, one or more perforated molded bodies or dies are immersed in the slurry in a felting tank containing an aqueous fiber dispersion. By applying a vacuum to the interior of the molded bodies, the fibers are caused to simultaneously and evenly “grow” on the molded bodies, creating in this manner the structured outer contour of the filter jacket. The known ring-shaped structures appear as depressions in the filter jacket and, hence, in negative form. Because of the already uniform density pattern of the structures, imparting a special functionality is then not possible.
A relatively long time is taken until an appropriate fiber volume grows via the slurry process. Also, the slurry process can generally only be implemented in an elaborate manner.